1. Field of the Invention
The invention is in the field of radiation detectors of the Geiger-Mueller type.
2. Description of the Prior Art
The Geiger-Mueller (G-M) counter is a simple and inexpensive detector for many species of ionizing radiation, such as x-rays, .gamma.-rays, .beta.-particles, and .alpha.-particles. Such an instrument typically consists of a gas-filled detector tube, a high voltage bias supply, and means for indicating the occurrence of an ionizing event within the detector tube, together with associated circuitry. For a general discussion of G-M counters see, for instance, A. C. Melisinos: Experiments in Modern Physics, Academic Press, 1966, pp. 175-177. Various types of detector tubes are in existence, but a very common design consists of a cylindrical chamber with a wire stretched along its axis. The chamber wall typically is grounded and acts as the negative electrode or cathode, whereas positive voltage is applied to the axial wire, the anode. The tube is filled with gas, such that ionizing radiation that enters the tube, typically through a "window", has a substantial probability of creating there one or more ion-electron pairs. Such an ionizing event we will call a "primary" event. If a sufficiently high voltage, of the order of one kilovolt, is applied to the anode then the free electrons that were created by a primary ionization event will be sufficiently accelerated towards the anode wire to produce secondary ion-electron pairs by collision, before eventually being collected at the anode. Similarly, the thus created secondary electrons can produce tertiary ion-electron pairs, and so on. For simplicity's sake, we refer to all non-primary ionization events as "secondary", and the products thereof as, e.g., "secondary electrons". If the tube is operated in the so-called "Geiger-Mueller regime" then a primary event will lead to substantial ionization of the volume of gas enclosed in the tube, and a discharge takes place. During the discharge the resistance between anode and cathode becomes negligible, and the tube acts essentially as a closed switch between the high voltage bias source and ground. The positive ions, being much more massive than electrons, require a substantial amount of time to drift to the cathode, where they eventually are neutralized, whereas the highly mobile electrons require only little time to reach the anode. External to the tube the discharge is observable as a current pulse having a short rise time, of the order of microseconds, and a long decay time, of the order of a millisecond. If a second primary ionization event should occur before all the ions produced as a consequence of a first primary event have been collected at the cathode then the resulting current pulse will be smaller than it would have been in the absence of the first event. This is so, inter alia, because the effective potential between anode and cathode is lowered by the presence of the positive ions. As long as primary events occur in sufficiently rapid succession the effective voltage between anode and cathode will not recover to the high level necessary for operation in the G-M regime, and the effective voltage may in fact become so low that substantially no pulses occur. In such a condition the G-M counter will appear to indicate a low or zero radiation level even though the detector tube is exposed to a high radiation level. It is obvious that such a condition would be a highly dangerous one for personnel relying on the G-M counter as an indicator of actual radiation levels present.